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st: xtmixed and random effects

From   Hubert Roth <>
Subject   st: xtmixed and random effects
Date   Thu, 16 Sep 2010 07:17:36 +0200

Dear all,
I have to analyze the data of a clinical trial comparing 3 meals eaten by the same subjects :
14 subjects come to have a meal at noon where food is "ad libitum" and quantity swallowed (energy, glucids, lipids, proteins, all continuous) is assessed every minute until the end of the meal. Duration is free and is varying from 15 to 31 min, according to the subject and the meal.
Each subject does this 3 times : once fasted since the day before (meal=0) and twice non fasted (meal = 1 or 2)
Initial energy (kcal) swallowed at t=0 is of course 0, so I chose "noconstant" for the intercept.
It seems to me that meal (0, 1 or 2) is a fixed effect, time too and subject is a random one, so I chose the following model :

xi: xtmixed kcal i.meal minute, noconstant || num_sujet: minute, cov(unstructured)

Am I right ?
Do I have to add a second random effect : "|| num_sujet: i.meal, cov(unstructured)" ?

Thanks for your help. Best regards,

Hubert ROTH, engineer, biostatistician
Centre de Recherche en Nutrition Humaine Rhone-Alpes
Rhone-Alpes Research Center for Human Nutrition
BP217 - F38043 Grenoble cedex 9 - France

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